EL display devices, which use a light emitting material that exhibits electroluminescence (hereinafter abbreviated as “EL”), have been attracting attention as display devices having a higher response speed and wider viewing angle than liquid crystal display devices.
An EL display device includes, for example, a TFT substrate, which is obtained by forming thin film transistors (TFTs) on a base made of a glass substrate or the like, and an EL element provided on the TFT substrate so as to be connected to the TFTs.
However, the EL element is generally easily affected by moisture, oxygen, etc., and reaction with a small amount of moisture or oxygen causes deterioration in their characteristics and reduction in lifespan of the display device.
Accordingly, a technology for preventing moisture and oxygen from entering the EL element by, for example, forming a sealing film on the EL element so as to seal the EL element has been known.
The sealing film may be formed by, for example, vapor deposition. The thicker the sealing film, the greater the blocking effect for preventing moisture and oxygen from entering the EL element. However, when the film thickness of the sealing film is increased, the film stress of the sealing film increases, and there is a risk of film separation, that is, a risk that the sealing film will be separated from the TFT substrate. There is also a risk that cracks will be formed in the sealing film when an external force is applied to the EL display device.
When the separation of the sealing film occurs or when cracks are formed in the sealing film, moisture, oxygen, etc., may enter the EL element through the separated or cracked portion and cause a light emission failure that causes, for example, dark spots or shrinkage.
PTL 1 describes a light emitting device in which an inorganic insulating film, which is provided on an organic EL element and serves as a sealing film, a stress absorbing film containing an organic compound, and an inorganic insulating film are arranged in that order. In the light emitting device described in PTL 1, the stress absorbing film, which has a stress less than those of the two inorganic insulating films, is disposed between the inorganic insulating films. Accordingly, the film stresses of the inorganic insulating films are absorbed, and separation of the inorganic insulating films is prevented.
PTL 2 discloses a gas barrier film including a SiOx thin film formed on a base film. A compressive residual strain is applied to the thin film to increase the breaking strain of the SiOx thin film and reduce the occurrence of cracks.
PTL 3 describes an organic EL display device in which a buffer layer is provided on a resin film, and in which TFTs, an organic EL element, and a sealing film are formed on the buffer layer in that order by, for example, vacuum deposition. The buffer layer of the organic EL display device according to PTL 3 has a multilayer structure in which a compressive stress layer composed of an inorganic film, a tensile stress layer, and a compressive stress layer are stacked in that order. The compressive stress layers and the tensile stress layer cancel each other's stresses, and thereby reduce the occurrence of cracks in the TFTs.